5 of us traveling to Mainland Spain in Late May so I was thinking about visiting one of these places. Its ,again, a want not a need your thoughts?
Ibiza is one relatively small island, with pretty, Mediterranean coastal scenery (but nearby Mallorca is more dramatic) and a world-famous party scene. If you're after the latter, there's no question, in any other case I'd pick Mallorca over Ibiza.
As for the Canary Islands... Well, first you have to pick one, perhaps 2, but no more. And they're much farther away, wilder (think 'volcanic'), more "unique"...but not worth heading all the way there for fewer than 5-6 days, whereas a 3-day stay in Mallorca or Ibiza is enough for a good overview/hangover.
Ibiza is one of those places young people go to "be seen." Pretty wild bar scene.
Canary Islands are just so far off the coast. It's one of those places people might stop on a repositioning cruise.
There's so much to see in Spain as it's a quite large country. We hit Madrid, Granada and Barcelona last June and enjoyed those cities. There's also a northern coastline most travelers to Spain never see.
We previously hit the southwestern coastline of Spain, and didn't care for the sea of high rise condos and congestion there.
More people visit the Canaries directly from Europe rather than via a repositioning cruise by a factor of about a million!
The Canaries vary significantly. Tenerife is the most developed, probably followed by Gran Canaria. Fuerteventura has the best sandy beaches and has the most unusual landscape, being dry and volcanic (non active). Lanzarote has fewer good beaches, but is popular. There are some smaller islands that are harder to get to, requiring a ferry from Tenerife.
I like the north of Fuerteventura for the beaches - I stay in Lajares when I visit.
There is more to Ibiza than partying, particularly in May.
Check out the flights from wherever you are on the mainland.